Mass spectrometry



Dec. 8 1953 L. P. ROBINSON EI'AL 2,662,185

MASS SPECTROMETRY 2 sheets sheet 1 Filed June 28, 1951 LELAND P.ROBINSON HAROLD W. WASHBURN WALTER J. H/RSCHBERG INVENTORS.

ATTORNEY Dec. 8, 1953 Filed June 28, 1951 FIGS.

gCOLLECTOR l3 33w if 30 SCOLLECTOR I3A HIGH CURRENT COLLECTOR I3 {32HIGH CURRENT AMPLIFIER LOW CURRENT LOW CuRRENT L. P. ROBINSON ETAL MASSSPECTROMETRY 4 SERVO 50 44 AMPLIFIER 3 6 3 M37 gm Fla. 2. 43

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RECORDER 52 HIGH CURRENT 28 HIGH CURRENT AMPLIFIER LOW CURRENT @COLLECToR I3A SERVO 50 LOW CURRENT AMPLIFIER 2 Sheets-Sheet. 2

MOTOR 58 REFERENCE COMPENSATION I/OL T4 GE PAPER MOVEMENT) RECORDER 52LELAND P. ROBINSON HAROLD W. WASHBURN WALTER J. HIRSCHBERG INVENTORS.

A T TORNEY Patented Dec. 8, 1953 UNITED STATES PATENT OFFICE MASSSPECTROMETRY ration of California Application June 28, 1951, Serial No.234,688

9 Claims. (01. ZED-41.9)

This invention relates to mass spectrometry and particularly to improvedmethods and apparatus for measuring isotope ratios.

In the practice of mass spectrometry a sample to be is first ionized, asby bombardment, with an electron beam. The resulting ions are segregatedaccording to their rnass-to-charge ratio, i. e. specific mass, and theions of a given specific mass are selectively discharged. lihe currentproduced by such selective ion discharge gives a measure of the partialpressure in the sample of the parent molecules from which the particularions derived.

In terms of apparatus, a mass spectrometer generally includes an ionsource, an analyzer chamber, and a collector electrode together withassociated power supply circuits, amplification and sensing circuits,and the like. The sample is introduced into the ion source wherein it isionized and the ions are propelled from the source into the analyzerchamber as a heterogeneous beam. In the analyzer chamber theheterogeneous beam is subjected to a transverse magnetic field wherebyions of differing 1nass-tocharge ratio are caused to follow differentpaths through the chamber. A single resulting beam may be focused on thecollector electrode or a plurality of beams may be successively focused.on the collector by varying the parameters determinative of the ionpaths through analyzer.

Recently mass spectrometers have been adapted to measurement of isotoperatios by inclusion therein of a pair of collector electrodes. One ofthese electrodes referred to as the low current electrode and at whichthe ion beam derived from molecules of a low abundance isotope isdischarged. The other elec'rode is referred to as the high currentelectrode and the ion beam or beams of the more abundant isotope orisotopes are focused on and discharged at this electrode. For example,consider the analysis of a sample of carbon dioxide to determine therelative abundance of the C and C isotopes. C being the most abundant,the C02 ions will he focused on the high current collector and the C02ions will be focused on the low current col lector.

In isotope ratio analysis it is generally desired to measure the ratioof the ion currents rather than the absolute value of each or" thecomponents. Accordingly, the amplification and recording circuitconnected to the two collector electrodes is often arranged as a nullbalance network, wherein the current developed at one collector iseffectively divided by the current deiii 2 veloped at the othercollector to give a recorded value representative of the indicatedratio.

Collector circuits of this character are illustrated and described insome detail in United States Patent 2,456,426 issued to Alfred O. C.Niel on December 14, 1948, and in arc-pending United States PatentApplication, Serial No. lee-.630, filed July ll, 1949 by Robert L. Sink.In the network described in said co-pending application, first andsecond amplifiers are connected to the high and low current electrodesrespectively, a potentiometer is connected across the first amplifierthe tap of the potentiometer is connected to the input resistor or thesecond amplifier. A servo system is connected to be operable responsiveto the output or the second amplifier to adjust the tap of thepotentiometer to the point there is no output from the second amplifier.Etecording means record the exe .rsions oi he poten tiometer tap inmaintaining the null balance.

In this system a signal is fed from the high current amplifier throughthe potentioine" er to the input of the low current amplifier iorthepurpose of balancing the current fed to the low current amplifier fromthe low current collector. When there is any change in the relativeabundance of the ion striking the two collectors, the balance in thesystem will be disturbed so that a signal appears at the output of thelow current amplifier. This signal drives the servo system to adjust thepotentiometer tap to return the tern to a null condition.

It has now been found that pressure variation in the ion source may havean appreciable effect on the isotope ratio measurement independent ofisotopic abundance. For example, if it is desired to continuouslymeasure and record the con centration of deuterium in water the ratio ofdeuterium to the sum or deuteri in plus hydrogen is recorded. Inpractice the desired r suit is accomplished by measuring the rati oi m/e19 to m/e 18 ion currents generated by ionization of the water sample inthe ion source. The significant contribution to the ni/e- 19 ion currentis HDO; the significant contribution to the m/e 18 ion current is H2O.

As an undesirable by-product of the ionization of the water sample, H3Oions are generated. These ions, which are termed isobaric ions, have thesame specific mass as do the ions; therefore, in order to obtain a trueindication of terium concentration, the contribution of these isobaricions must be compensated for by some method. It has been found that theion current produced by these isobaric ions is proportional to thesquare or the partial pressure of water vapor within the ion source; theerror introduced in the sensed ratio being therefore proportional tothis partial pressure.

It has been proposed as a means of eliminating the error factor due toisobaric ions that the total inlet sample pressure ahead of themolecular leak into the ion source be maintained very nearly constant,as for example at 100 microns plus or minus 2 /2% and electricallysubtract out the isobaric ion contribution to the desired ratio. Thismethod has a number or disadvantages. Apparatus for maintaining totalinlet pressure within the allowable tolerance is extremely expensive.The method contemplates only regulation of the total pressure. If thereshould be any significant quantities of impurities in the sample, thepartial pressure of water vapor will vary with variations in impurityconcentra= tion, and since the isobaric ion contribution is a functionof the partial pressure of the water vapor, error may be introducedindependent of total pressure variation. Moreover, the error re sultingfrom the presence of isobaric ions is, as mentioned above, proportionalto water vapor pressure inside the ion source and rigorous control ofthe pressure on the downstream side of the sample leak will notnecessarily result in equally rigorous control of the pressure withinthe ion source. It is quite possible that variations in leak rate mayoccur, as for example due to partial plugging of the extremely smallleak orifice, or that pressure in the ion source may vary independentlyof sample pressure due to temperature fluctuations or other factors.

In accordance with the present invention we propose to sense variationsin partial pressure of water within the ion source itself rather than bymeans of a separate external transducer, and to accomplish this sensingbased on m/e 13 ions. The magnitude of the m/e 18 ion current asmeasured by the output voltage of the corresponding amplifier, is afunction of various parameters, as follows:

(a) Directly proportional to the partial pres sure of water vapor withinthe ionization region of the ion source;

(b) Directly proportional to the intensity of the ionizing electronbeam;

Directly proportional to the resistance of the input resistor of therespective amplifier;

(d) Directly proportional to the gain of this amplifier; and

(6) some undetermined function of the relative potentials applied to thevarious electrodes in the ion source.

Under normal operating conditions, the last four of the above listedparameters will be held essentially constant over at least a 24 hourperiod so that magnitude of the in/e 18 ion current becomes dependentsolely on the partial pressure of water vapor within the ionizationchamber. As a result, variations in the output voltage of the highcurrent amplifier will be an accurate measure of variations in thepartial pressure of water vapor within the ionizing region of the ionsource.

The present invention contemplates utilization of the variation inoutput voltage of the amplifier connected to the high current collector,i. e. the collector on which the m/e 18 ions are discharged to apply acompensating factor to the ratio measurement of the null balancenetwork. This method is applicable to eliminate error due to isobaricions when the ratio between the iso topes to be measured is extremelylarge, as for example where the total concentration of HDO in the wateris 0.1% or less. When relative concentration of HBO and H20 is less thani; of 1%, a significant change in HDO concentration does not appreciablyvary the H20 concentration so that for all practical purposes any changein partial pressure of water vapor will not be as a result of change inconcentration of H20 but will be due to fluctuations in inlet pressureor the variations in impurities in the sample. As a result, anyvariation in the H20 partial pressure, as reflected in variation in theoutput of the high current amplifier, will be reflected in variation ofthe abundance of isobaric ions, and the total variation in the amplifieroutput may be used to determine the applied correction factor.

The invention therefore contemplates in mass spectrometry whereinisotope ratios are determined by ionizing a sample, segregating ions ofthe isotopes to be determined, separately discharging the ions of theseisotopes and determining the ratio of the discharge currents therebyproduced, the improvement which comprises separately sensing variationsin one of the, discharge currents and, varying the determined ratio inproportion to such variation,

In another aspect the invention contemplates in an isotope ratio massspectrometer having dual ion collectors and electrical means fordetermining the ratio of discharge currents developed at the collectors,the combination comprising means operable responsive to variation in thedischarge current at one of the collectors to vary the ratio inproportion to such variation.

Several means may be employed to introduce the compensating factor tothe null balance net= work. The simplest of these means comprises manualadjustment of a compensating resistor in proportion to variation of theoutput of the high current amplifier. This means is relativelyunsatisfactory, particularly where continuous analysis is sought. Inpreferred practice compensation is carried out automatically, as forexample by means of a servo system operative responsive to thevariations in output of the high current amplifier to vary the settingof a compensating resistor in the null balance network or by means of anelectronic amplifier connected to feed an auxiliary current through afixed resistance located in the null balance network, the magnitude ofthis auxiliary current being proportional to the difference between thehigh current amplifler output voltage and a reference voltage.

The invention will be more clearly understood by reference to thefollowing detailed description thereof taken in conjunction with theaccompanying drawing, in which:

Fig. 1 is a diagrammaticillustration of one type of mass spectrometer;

Fig. 2 is a circuit diagram 01 one embodiment of the invention, and

Fig. 3 is a circuit diagram of a second embodiment of the invention.

The conventional magnetic analyzer mass spectrometer showndiagrammatically in Fig. 1 has an ion source ii, an analyzer tube l2 anda pair of ion collectors l3, 43A, all disposed within an envelope It.Analyzer tube [2 is provided at the end adjacent the ion collectors It,MA with so-called resolving or exit slits 45, Hill, respectively,through which the ion beams are focused on the respective collectors. Anelectron gun (not shown) develops an electron beam it which is projectedthrough the ion source between a pusher electrode H or a pair of pusheror repeller electrodes and a first accelerator electrode 13. The firstaccelerator electrode i3 and a sec- 0nd accelerator 2d spaced therefromare provided respectively with a first slit 18A and a second slit 26Athrough which ions formed by the beam it are collinlated and propelledinto the analyzer tube 12.

The spectrometer is provided with an envelope exhaust line 24 which maybe connected to a mercury diffusion pump, molecular pump or anyappropriate evacuating system (not shown). The analyzer tube may beprovided with ports 25 by means of which the ion source and the analyzertube are evacuated through the envelope. A1- ternatively, the envelopemay be omitted by maizing the analyzer tube l2 gas-tight and attachingthe evacuating system to it by means which are well known in the art.

An inlet line 25 provides means for introducing the sample to beanalyzed either continuously or intermittently into the ion source H.The ions formed in the source are propelled therefrom as a heterogeneousbeam into the analyzer tube 12. In the analyzer tube the heterogeneousbeam is segregated into diverging ion beams, each beam being composed ofions of a given specific mass, the ion segregation being brought aboutby means of an electrical or magnetic field established across the tubetransverse to the direction of ion travel. Means for obtaining such atransverse field are well known and form no part of the presentinvention and may comprise magnet pole pieces (not shown) disposedadjacent opposite walls of the tube. The diverging ion beams are focusedon the ion collectors i3, its through the exit slits i5, IEA byadjusting the potential applied to the accelerating electrodes ll, it.In the specific example considered, HDO ions as well as any isobaricions formed in the system will be focused through the exit slit ISA tofall on collector [3A, and H20 ions will be focused through exit slit IEto strike the collector l3. The ions of the heaviest mass assume pathsof travel of the greatest radii. The collector electrodes i3, HA areconnected to an amplification and recording system (not shown in Fig.1), by leads 28, 23A, respectively, sealed through the tube Hi.

The mass spectrometer illustrated in Fig. l is only one of manyconventional types. The invention is directed to improvements in massspectrometers of any type capable of measuring the relative abundance ofisotopes in a sample. In accordance with the invention, the leads 2%,28A are connected to an amplification and recording system of the typeshown in Figs. 2 or 3.

Referring to Fig. 2, the high current collector [3, this being thecollector on which the more abundant H2O ions are discharged, isconnected to a feed-back type D. C. amplifier 36. The feedback system ofthe amplifier is shown diagrammatically only and includes a lead 3|connecting output lead 32 with the input lead 23 through an input gridresistor 33. The low current collector 13A is connected by lead 23A to aD. C. ampliher 36 provided with a zero set adjustment 3?. The outputlead 32 of the high current amplifier 3l is connected to one end of avoltage dividing network it including resistors 4!, 42 and a tapped-elfresistor 43 serially connected between the lead 32 and ground. Aslidewire potentiometer 44 is connected to the voltage dividing network46 across the resistor 42. The tap MA of the potentiometer 44 isconnected by a lead it to the input lead 28A of amplifier 36 through aninput grid resistor 48. The output side of the amplifier 36 isconnectedto drive a servo system 3, 50 which in turn is connected tocontrol the osition of the tap 44A along the slidewire potentiometer 44.A recorder 52 is ganged to the system to record the excursions of thepotentiometer tap A. To this extent, and with the exception of thearrangement of the voltage dividing network 4d, the system issubstantially the same as that described in the prior art for thispurpose.

Before the ion current is turned on, i. e. before the current is beingdelivered from the collector electrodes to the amplifiers, amplifier 35is adjusted for zero output by means of the zero set control 3?. In thecircuit as illustrated, it should be pointed out that zero balanceresults in the pen moving at zero rate regardless of position.

With amplifier 36 balanced, the ion current is turned on and the circuitautomatically adjusts itself to indicate the correct isotope ratio. Thesignal applied to amplifier as from collector i3 is applied to thevoltage divider network id with-- out voltage amplification, sinceamplifier 3G is a feed-back amplifier having a unity gain. After themagnitude of the output signal from amplifier 36} is adjusted in thevoltage divider dil it is applied across the potentiomet r M. A portionof this signal is tapped-off by tap MA and applied through grid resistorto the input of amplifier 35. When the signal tapped oif thepotentiometer M and applied to the input of the amplifier 36 exactlybalances the signal from the low current collector iiiA, the output ofthe amp-lifier 35 will be at zero level. There will then be no signalapplied to the servo system and the tap MA will remain at a fixedposition. When the ion current at EBA varies, the resultant un balanceoutput of amplifier (it will actuate the servo system to vary thesettling of tap 54A to rebalance the network.

In accordance with one embodiment of the present invention, as shown inFig. 2, a servo sy-- tem is connected to the output lead of the highcurrent amplifier Till to adjust the position of the tap 53A on theresistor 43 in the voltage dividing circuit 46 in proportion to thevariations in the amplifier output. This system includes a servoamplifier 54 into which the output of amplifier 30 and a referencevoltage signal are fed. The reference voltage is derived from a voltagesource 55 connected across a sliclewire potentiometer 5G, a fraction ofthe voltage across the potentiometer 55 being tapped therefrom and. fedto the servo amplifier a l. The amplifier is so constructed as toamplify only the difference between the output voltage of the highcurrent amplifier and the reference voltage, the amplified difierencebeing applied to drive a servo motor which is connected to adjust thepositions of tap dbl l, alon resistance as and tap of the poteniometer56. The effect of the variation of resistance between resistor t2 andground in the voltage divider network such variation being brought aboutby variation in the portion of the resistor 43 tapped-off by the tapsci-i, is to the potential of the X-end of the potentiometer st withrespect to ground, i. e. to vary the potentiometer to ground potential.If the error factor was of any appreciable magnitude, a second variableor compensating resistor would have to be located between the resistorii and the potentiometer ii to avoid a change in voltage across thepotentiometer. since the error factor is small as compared to thevoltage across the potentiometer the additional compensating resistorcan be eliminated and for all practical purposes the effect of theadjustment of resistor 43 is to shift the potentiometer to groundpotential without varying the voltage acrossthe potentiometer. In otherwords, the voltage change acrossthe slidewire is negligible, theimportant result of the compensatingnetwork being to vary the voltage atthe K-end or the potentiometer with respect to round,

As fully described above, the efiectxofeintroducs this mpensatingfunction into the:ratiomeasuring network is to balance out :any spuriousvariation in ratios brought-about byfiuotuation in the partial'pressureof Water .vaporin the ion source, which fluctuation causes aproportional variationin the presence of objectionable isobaric ions.

Another means .for accomplishing the :same end result is shoWninFigUE.The circuit shown in Fig. 3 is, inmany respects identical to that shownin Fig. 2, including the. amplifiers;3fl,i.38, feed-back loop 3I throughgrid resistor tothe amplifier 3o andthefeed-back loop 46 frompotentiometertap 44A and through grid resistor'lfi to the input oftheamplifier 316. The outputf32 of the amplifier is again-.connected toone .end of a voltage dividing network :40 including resistances t I, 42and d3. serially connected between the outputlead 32 of the amplifier30am! ground and a slidewire potentiometer l l connected across theresistor 42. In thisembodiment, accompansating functionxis. applied tothe ratio-.measuring system by means of a direct current electronicamplifier network includingAa comparator tube 60 and a cathodefollowerliZ. :In the'comparator tube 50 a signal is developed intheplate circuit proportional to the difference between a referencesignal derived from'a voltagessource v(i l and afraction of theoutput'signaliof'the'amplifier 30 as tapped-oif aslidewirexfifi. Theztwosignals are applied to the grids of the comparator tubetfl and theunbalance signal, if any,.isapplied-to the grid of thecathcdefollower-62. :With thissystem, when the output'of the high currentamplifier varies so that there isaniunbalance in the signals applied tothe gri'dtfl, a currentis introduced to the voltage divider network 46at'the point Y between the-resistors 42 and :so asto vary the voltagedrop across the network and shift the potential across the potentiometerM withrespect to ground. Asdnrthe-system of Fig. 2, this shift isaccomplished without 'ISlgIllfiCfi-Ilt change in the voltage acrossthepotentiometer, the current fiow upwardly inthevoltage divider fromthe-point of introduction at Y being negligible in comparison withthatflowing through resistor 43 to ground.

The functioning of the system 'ofFig. 3 is substantially identical withthat of the system of Fig. 2, the net result being to cancel out anyerror introduced by fluctuations in partial pressure of water vaporinthe: ion source.

Ihe invention has been described 'particulan ly with reference totheanalysis of HBO-H2O ratios but is equally applicable to :any isotopicratio-measurement in which therproblem cruise-- baric ions isencountered. 'Asanadditional ere ample of the existence :of this problemis the analysis of the ratioo'f 'Hz-HD isotopes, wherein H3 is presentan isobaric ion toan extent sen sible to variations in partial pressureof'Hz in the ion source.

'W e claim:

1. In an isotope ratio mass spectrometer having dual ion collectors andelectrical means for I determining the ratio of the discharge currents 8to vary the indicated ratio in proportionto .said variation.

2. In an isotope ratio mass-spectrometer having dual ion collectors andelectrical meansfor determining the ratio of the discharge currentsdeveloped at the collectors, the combination comprising means operableresponsive to variation in the discharge currentat the one of saidcollectors on which the more abundant isotope is discharged to varytheindicated'ratio in proportion tosaid variation.

3. In a mass spectrometer comprising two collector electrodes, a firstamplifier connectedmo one of the electrodes, a'second amplifierconnected to the other. of the electrodes, a potentiometer connectedbetween the first amplifier and ground, the tap of'the potentiometerbeing connected to the input resistor of thesecond amplifier, and meansoperable responsiveto output of the second amplifier to adjusttheposition of the potentiometer tap to drive the second amplifier to a:null outputythe improvement comprising means operable responsive tovariationin the output of. the first amplifier to apply a signalbetweenthe potentiometer and ground proportional to such variation.

4.121 a mass spectrometer comprisingtwocollector electrodes, a firstamplifier connected to one of 'the .electrodes,-a-second amplifier connected to the other of the electrodes, a potentiometer connected betweenthe first amplifier and grounrhthe tap of the potentiometer beingconnected to the input resistor ofthe second amplifier and meansoperable responsive tothe output of the second amplifier to adjust theposition of the potentiometer tap, the improvement comprisingmeansoperable responsive to-variation in the output of thefirst'amplifier to vary the potentiometer to ground potentialandproportionaltto said variation in the first amplifier output.

5. Apparatus according to claim 4 wherein said means for varying .thepotentiometer to ground potential comprises a variable resistorconnected between the potentiometer and ground and ineansfor varying thevalue of the resistor in proportion to variation in the output of thefirst amplifier.

6. Apparatus'according to claim 4 wherein said means for varyingthepotentiometer to ground potential comprises'a variable resistorconnected between the potentiometer and ground, aservo amplifierconnected to the output of said first amplifier in parallel-with thepotentiometer, a source of reference voltage connected to said servoamplifier whereby the output of the-servo amplifier is proportional to.the algebraic sum of said reference voltage and first amplifier output,and a servo motor operable responsive to output of saidservo amplifierto vary'said'variable .resistance and the reference voltage.

'7. Apparatus according to claim A "wherein said means for varying the'potentiometer to ground potential comprises a fixed resistanceconnected serially between the; potentiometer and ground and .a directcurrent amplifier connected to the output of the first amplifier inparallel with said potentiometer, asource of reference voltage connectedtothe directcurrent amplifier so that the output thereof is proportionalto-the algebraic sum of the reference voltage and the output of thefirst amplifier, and-meansfor feeding an additional current through thefixed'resistor and proportionalto the outputof the direct currentamplifier.

8. In a mass spectrometer having'two ion "col- 5i lector electrodes, afirst amplifier having its input circuit connected to one of theelectrodes, a second amplifier having its input circuit connected to theother electrode, and means coupled to the output circuits of the twoamplifiers for providing a comparison of the relative magnitudes of thedischarge currents developed at the two collectors, the improvementwhich comprises means in addition to the comparison means coupled to theoutput circuit of the first amplifier and responsive to variations inits output signal for varying the amplification of the second amplifier.

9. In a mass spectrometer having two ion collector electrodes, a firstamplifier having its input circuit connected to one of the electrodes, asecond amplifier having its input circuit connected to the otherelectrode, a voltage divider connected across the output circuit of thefirst amplifier, an adjustable tap on the voltage di- References Citedin the file of this patent UNETED STATES PATENTS Name Date Nier et a1Dec. 14, 1948 Number

